AC CREEPAGE DISCHARGES IN SF 6, CO 2, N 2 AND SF 6 -CO 2 AND SF 6 - N 2 MIXTURES

Transcription

1 AC CREEPAGE DISCHARGES IN SF 6, CO 2, N 2 AND SF 6 -CO 2 AND SF 6 - N 2 MIXTURES F. Sadaoui and A. Beroual* Ecole Centrale de Lyon, AMPERE CNRS UMR 5005, 36, Avenue Guy de Collongue, Ecully, France * <Abderrahmane.Beroual@ec-lyon.fr> Abstract: This paper deals with the experimental study of creepage discharges propagating over insulators made of different materials namely Glass, Epoxy resin and Bakelite, under AC voltage. The mainly investigated parameters are the morphology of discharges and their stopping lengths L f versus the voltage magnitude, the type and thickness of insulator, the type of gas/gaseous mixture and pressure. The gases and mixtures we considered are N 2, SF 6, CO 2, SF 6 -N 2 and SF 6 -CO 2. It s shown that L f increases quasi-linearly with the voltage; it decreases when the thickness is increased and/or the dielectric constant of insulator decreases, thus indicating the important role of electric field and capacitive effects in the propagation mechanism. On the other hand, the increase of SF 6 content in SF 6 -CO 2 and SF 6 -N 2 mixture leads to a significant decrease of the length of discharges. 1 INTRODUCTION From insulation viewpoint, the triple junction constitutes the weakest point in any high voltage equipment (bushings, insulators, switchgears, gas insulating lines, gas circuit breaker ). Indeed, when the applied voltage, and then the electric field, reaches a critical value, partial discharges can be initiated in the vicinity of this region. The increase of the voltage leads these PDs to develop and to transform in surface discharges (creeping discharges) that propagate over the insulators up to flashover. This results in partial or total deterioration of the insulation system requiring the repair of equipments or even their replacement. To avoid such situations, the designers of high voltage apparatus have to know the characteristics of these discharges and their evolution depending on the type of insulator and gas as well as their physical and geometrical properties, the electrodes geometry and the voltage wave form as well as its amplitude and time application. Thus, the knowledge of the optical and electrical parameters of these discharges is of great interest for industrial applications. In previous papers, we investigated the characteristics of these discharges when propagating over insulators made of epoxy resin, glass and polytetrafluoroethylene (PTFE) filled with different kinds of micro- mineral fillers, immersed in gas or gaseous mixtures, under standard lightning impulse voltage (1.2/50 µs) [1-7]. This paper deals with the characterization of this type of discharges but under AC voltage, using a point-plane electrode arrangement. We mainly investigate their morphology and their stopping length against the voltage magnitude, the type of insulator and its thickness, the type of gas/gas mixture and its pressure. 2 EXPERIMENTAL SETUP The experimental arrangement is similar to that we used in previous work (figure 1) [6, 7], except the voltage supply which is here a 50 kv/50 Hz transformer. For the continuity, we recall here its main features. kv HV source ma Voltage regulator Camera Resistance Figure 1: Scheme of experimental setup The test cell containing the considered insulating structure (solid/gas or mixture) and a point-plane electrode arrangement consists of a cylindrical core of 90 mm high and 110 mm inner diameter, and two flats and circular covers (Figure 2); the upper cover was of PMMA (transparent material) enabling to visualize the discharges and to support the sharp electrode; the lower one which constitutes also the plane electrode is a circular plate of 250 mm diameter and 15 mm thickness, was of brass. The point electrode was made of tungsten, the radius tip of which is 10 µm. The cylindrical core of the test cell consists of two screwed parts: an upper part of 60 mm high made PC Test cell Oscilloscope

2 of Teflon and a bottom part issued from Plexiglas of 30 mm high enabling to control the contact between the point electrode and the solid insulator. The solid insulating samples are discs of 100 mm diameter and 2 to 10 mm thickness. Three types of solid insulating samples were tested: Glass, Epoxy resin and Bakelite. The gases and mixtures we considered are N 2, CO 2, SF 6, SF 6 -N 2 and SF 6 -CO 2. The solid samples are changed each time we observe traces on their surface and the radius of curvature of electrode point is checked before and after testing. H T Point Electrode (1) (2) applied voltage (Figure 3) and decrease when the gas pressure is increased (Figures 4). For a given gas pressure, the increase of stopping lengths of creeping discharges L f is quasi-linear with the voltage (Figures 5 to 7). SF 6 0.3MPa 18kV CO 2 0.3MPa (a) SF 6-0.3MPa 22kV CO 2-0.3MPa Creeping discharge (3) (4) Plane Solid sample Gas (5) (6) (7) 14kV (b) 20kV (8) N 2-0.3MPa N 2-0.3MPa 1. Transparent cover (Plexiglas) 5. Solid sample 2. Admission / gas mixture 6. Transparent cylinder 3. Electrode tip 7. Plane electrode 4. Cylindrical body (Teflon) 8. Manometer Figure 2: Scheme of the test cell A system of taps associated to two manometers enables to fill the test cell and to control the pressure. When the gases are mixed, the gas with the lower partial pressure (SF 6 ) is first admitted into the test cell and then the higher partial (N 2, CO 2 ) is admitted after the pressure of SF 6 is stabilized. 14kV 21kV (c) Figure 3: Influence of the voltage magnitude on creeping discharges propagating over different insulators of 2 mm thickness, immersed in different gases at 0.3 MPa: (a) Glass in SF 6 ; (b) Glass in CO 2 ; and (c) epoxy resin in N 2. A CCD camera (SONY XC-HR58 type, high SVGA resolution (767x580 pixels)) connected to a high performance video card (Meteor-II/Multi-Channel) is used to capture the integral light image of discharges. Thanks to this optical system, one can measure the maximum extension of discharge L f directly on the computer screen. For a given voltage, the considered value of L f is the average of ten tests. 3 EXPERIMENTAL RESULTS SF 6-0.1MPa 20kV N 2 0.3MPa SF 6-0.3MPa 20kV (a) N 2 0.4MPa As observed under lightning impulse voltage, the shape and stopping length of discharges depend on the type and thickness of the insulator, the nature and pressure of the gas (or gas mixture), and the amplitude of applied voltage. 3.1 Influence of the amplitude of voltage, the type of gas and its pressure For given solid and gas pressure, the length of the discharge channels increases with the amplitude of 16kV (b) 16kV Figure 4: Influence of pressure on creeping discharges propagating at: (a) Glass/ SF 6 and (b) Epoxy resin/n 2 interfaces; e=2mm.

3 For a given pressure and voltage, L f is longer with CO 2 and N 2 than with SF 6 (L f (N 2 ) < L f (CO 2 ) < L f (SF 6 )) whatever the type of insulator. Similar results have been obtained under standard lightning impulse voltage with insulators made of glass and resin epoxy [1, 3, 5, 6]. strongly on the thickness of insulator; it decreases when the thickness increases. On the other hand, for a given voltage, L f is higher with Bakelite (ε r =4.8) and Glass (ε r =5) samples than with Epoxy samples (ε r =3.5). This is due to the electric field at the point which is more important as the difference of the permittivity between the solid insulating sample and gas is high. Figure 5: Stopping length of creeping discharges over glass sample vs. the voltage for different pressures of the investigated gases, e= 2mm. 3.2 Influence of the nature and the thickness of solid insulating The thickness and the nature of solid insulating sample also influence the shape and final length L f of discharges. Figure 8 gives an example of discharges propagating at SF 6 /glass and SF 6 /Bakelite interfaces for different thicknesses of insulators. The total density of branches depends Figure 6: Stopping length of creeping discharges over Epoxy resin sample vs. the voltage for different pressures of the investigated gases, e= 2mm. On the other hand, when the thickness of insulator increases then the capacitance of the system consisting of the insulator inserted between the electrodes decreases, Lf decreases (Figures 9 and 10).

4 The above observations (influence of the thickness of solid and its dielectric constant) indicate the important role of the capacitive effects and the electric field on the propagation mechanism. This phenomenon has been also observed with solid/gas interface under lightning impulse voltage [1-7] as well as with solid/liquid interfaces [8, 9] SF 6-0.3MPa 24kV e = 4mm SF 6 0.2MPa 24kV SF 6-0.3MPa 24kV e = 6mm (a) SF 6 0.2MPa 24kV e = 4mm (b) e = 6 mm Figure 8: Influence of thickness of insulator on creeping discharges propagating over: Bakelite /SF 6 interface (a); and glass/sf 6 interface (b). Figure 7: Stopping length of creeping discharges over Epoxy resin sample vs. the voltage for different pressures of investigated gases, e= 2mm. 3.3 Influence of SF 6 content The addition of small amount of SF 6 to CO 2 or N 2 reduces the discharge branches (Figures 11 and 12). For each gas mixtures, the stopping length of discharges also increases quasi-linearly with the voltage. Figure 9: Stopping length of creeping discharges over glass sample vs. the voltage and thickness of insulator for different pressures in N 2 and CO 2. Figures 13 and 14 show examples of evolution of L f vs. the voltage for glass immersed in SF 6 /N 2 and SF 6 /CO 2 mixtures for different concentrations of SF 6.

5 various elements constituting the mixtures. For a given interface solid/gas (mixture), we showed that L f increases quasi-linearly with the applied voltage and reduces when the pressure of gas increases. CO 2/10%SF 6 CO 2/15%SF 6 20 kv 20 kv Figure 12: Influence of SF 6 content in CO 2 at 0.2MPa on creeping discharges propagating over glass. Figure 10: Stopping length of creeping discharges over glass sample vs. the voltage and thickness of insulator in N 2 and CO 2, for different pressures. N 2/5%SF 6 N 2/10%SF 6 16 kv 16 kv N 2/15%SF 6 16 kv Figure 11: Influence of SF 6 content in N 2 at 0.2MPa on creeping discharges propagating over glass. 4 CONCLUSION The results we got in this work are similar to those obtained under lightning impulse voltage. Indeed, the morphology and stopping length L f of discharges propagating over a solid insulator (Glass, Bakelite and Epoxy resin) immersed in various gases and mixtures (SF 6, N 2, CO 2, SF 6 -N 2 and SF 6 -CO 2 ) submitted to AC voltage, depend on the amplitude of the voltage, the type and the thickness of solid insulator, the nature and pressure of gas as well as the concentration of Figure 13: Stopping length of creeping discharges over glass immersed insf 6 /CO 2 and SF 6 /N 2 mixtures at 0.2MPa versus the voltage. For a given applied voltage and gas pressure, L f is longer with CO 2 and N 2 than with SF 6 (L f (N 2 ) < L f (CO 2 ) < L f (SF 6 )); it decreases when the thickness is increased and/or the dielectric constant of insulator is decreased indicating thus the important role of capacitive effect and the electric field in the propagation mechanism. On the other hand, the addition of small content of SF 6 reduces L f. indicating the role of electronegative gas.

6 Management Technology for Green Operation, September 11 14, 2010, Tokyo, Japan. [5] A. Beroual, M-L. Coulibaly, O. Aitken and A. Girodet, Study of creeping discharges propagating over epoxy resin insulators in presence of different gases and mixtures, 2010 International Conference on High Voltage Engineering and Application, October 11 th 14 th, 2010, New Orleans, USA [6] A. Beroual, M-L. Coulibaly, O. Aitken and A. Girodet, Investigation on Creeping Discharges Propagating over Epoxy Resin and Glass Insulators in Presence of Different Gases and Mixtures, The European Physical Journal Applied Physics, Volume 56, Issue 03, December 2011, p (8 pages). [7] A. Beroual, M-L. Coulibaly, O. Aitken and A. Girodet, Effect of Micro-fillers in PTFE Insulators on the Characteristics of Surface Discharges in Presence of SF6, CO2 and SF6-CO2 Mixture, IET Generation, Transmission and Distribution, October 2012, Vol. 6, Issue 10, 2012, pp Figure 14: Final length of creeping discharges over epoxy resin immersed in SF 6 /N 2 and SF 6 /CO 2 mixtures at 0.2 MPa versus the voltage. 5 REFERENCES [1] M-L. Coulibaly and A. Beroual, Experimental characterization of creeping discharges over solid/gas and solid gaseous mixture interfaces under lightning impulse voltage, XVI International Conference on Gas Discharges and Their applications, 7th 12th September, 2008, Cardiff, Wales, UK. [2] M-L. Coulibaly, A. Beroual, O. Aitken and A. Girodet, Investigation on Creeping Discharges Propagating over Insulators Immersed in Gases and Gas Mixtures under Lightning Impulse Voltage, 2008 International Conference on High Voltage Engineering and Application, Chongqing, China, 9 th 12 th November, [3] M-L. Coulibaly, A. Beroual, Creeping discharges propagation over insulators issued of different materials in presence of SF6, N2, CO2, SF6-N2 and SF6-CO2, 16 th International Symposium on High Voltage Engineering, Cape Town, South Africa, August [4] A. Beroual, M-L. Coulibaly, O. Aitken and A. Girodet, Influence of fillers in PTFE insulators on the characteristics of surface discharges in presence of different gases and mixtures, International Conference on Condition Monitoring and Diagnosis 2010 (CMD 2010) Modern Asset [8] L. Kebbabi and A. Beroual, Optical and Electrical Investigations on Creeping Discharges over Solid/Liquid Interface under Lightning Impulse Voltage, IEEE Trans. Dielectrics and Electrical Insulation, Vol. 13, No. 3, 2006, pp [9] A. Beroual, L. Kebbabi, E. A. Al-Ammar and M. I. Qureshi, Analysis of Creeping Discharges Activity at Solid/Liquid Interfaces Subjected to AC Voltage, IET Generation, Transmission and Distribution, Volume 5, Issue 9, pp , September 2011.